#include <stdint.h>

#define GPIO_BASE   0x40000000u
static volatile uint32_t * const R_FREQ     = (volatile uint32_t *)(GPIO_BASE+0);
static volatile uint32_t * const LEDS       = (volatile uint32_t *)(GPIO_BASE+4);
static volatile uint32_t * const LEDGR      = (volatile uint32_t *)(GPIO_BASE+8);

#define TIMER_BASE  0x40010000u
static volatile uint32_t * const TIMER      = (volatile uint32_t *)(TIMER_BASE+0);

#define MSTATUS_MIE (1u << 3)
#define MIE_MTIE    (1u << 7)

extern void trap_entry();

static inline void irq_init() {
    /* mtvec first */
    asm volatile ("csrw mtvec, %0" :: "r"(trap_entry));

    /* enable machine timer interrupt */
    asm volatile ("csrs mie, %0" :: "r"(MIE_MTIE));

    /* global enable last */
    asm volatile ("csrs mstatus, %0" :: "r"(MSTATUS_MIE));
}

void timer_isr(){
    static int set = 0;
    *TIMER = 1840000*2;
    *LEDGR = ~(*LEDGR);
}

void main(){
    irq_init();

    *LEDGR = 3;
    *TIMER = 1840000*2;

    for(;;){
        for(int i=1000; i<10000; i++){
            *R_FREQ = i;
            for(int j=0; j<80; j++) asm volatile("nop");
        }
    }
}